Zoological Science

The mitochondrion-rich cells (MRCs) in teleost gill and equivalent tissues are important osmo-regulatory sites in maintaining ionic balance. These cells express a variety of ion pumps, transporters, and channels, which play central roles in ionic regulation. Recently, two types of MRCs have been identified in euryhaline fishes: seawater (SW)-type MRCs extrude Na and Cl ions in SW conditions; freshwater (FW)-type MRCs take up at least Cl⁻. Long-term development/differentiation of the two types of MRCs during adaptation to different salinities appears to be regulated mainly by endocrine factors. Osmolality, Ca²⁺, neurotransmitters, and fast-acting hormones rapidly regulate the SW MRCs. Recent information is assembled in this review and suggests the functional plasticity of highly specialized MRCs.

Compound eyes, nauplius eyes, frontal organs, intracerebral ocelli, and caudal photoreceptors are the main light and darkness detectors in crustaceans, but they need not be present all at once in an individual and in some crustaceans no photoreceptors whatsoever are known. Compound eye designs reflect on their functions and have evolved to allow the eye to operate optimally under a variety of environmental conditions. Dark-light-adaptational changes manifest themselves in pigment granule translocations, cell movements, and optical adjustments which fine-tune an eye's performance to rapid and unpredictable fluctuations in ambient light intensities as well as to the slower and predictable light level changes associated with day and night oscillations. Recycling of photoreceptive membrane and light-induced membrane collapse are superficially similar events that involve the transduction cascade, intracellular calcium, and membrane fatty acid composition, but which differ in aetiology and longterm consequence. Responses to intermittant illumination and linearly polarized light evoke in the eye of many crustaceans characteristic responses that appear to be attuned to each species' special needs. How the visual responses are processed more centrally and to what extent a crustacean makes behavioural use of e-vector discrimination and flickering lights are questions, however, that still have not been satisfactorily answered for the vast majority of all crustacean species. The degree of light-induced photoreceptor damage depends on a large number of variables, but once manifest, it tends to be progressive and irreversible. Concomittant temperature stress aggravates the situation and there is evidence that free radicals and lipid hydroperoxides are involved.

Changes in branchial vacuolar-type H⁺-ATPase B-subunit mRNA and Na⁺,K⁺-ATPase α- and β-subunit mRNA and ATP hydrolytic activity were examined in smolting Atlantic salmon exposed to hyperoxic and/or hypercapnic fresh water. Pre-smolts, smolts, and post-smolts were exposed for 1 to 4 days to hyperoxia (100% O₂) and/or hypercapnia (2% CO₂). Exposure to hypercapnic water for 4 days consistently decreased gill vacuolar-type H⁺-ATPase B-subunit mRNA levels. Salmon exposed to hyperoxia had either decreased or unchanged levels of gill B-subunit mRNA. Combined hyperoxia+hypercapnia decreased B-subunit mRNA levels, although not to the same degree as hypercapnic treatment alone. Hyperoxia generally increased Na⁺,K⁺-ATPase α- and β-subunit mRNA levels, whereas hypercapnia reduced mRNA levels in presmolts (β) and smolts (α and β). Despite these changes in mRNA levels, whole tissue Na⁺,K⁺-ATPase activity was generally unaffected by the experimental treatments. We suggest that the reduced expression of branchial vacuolar-type H⁺-ATPase B-subunit mRNA observed during internal hypercapnic acidosis may lead to reduction of functional V-type H⁺-ATPase abundance as a compensatory response in order to minimise intra-cellular HCO₃ ^– formation in epithelial cells.

The possible involvement of nitric oxide (NO) in regulating the motile activities of teleostean melanophores was studied in the dark chub Zacco temmincki (Cyprinidae, Cypriniformes) and in the translucent glass catfish Kryptopterus bicirrhis (Siluridae, Siluriformes). NO donors, including (±)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexaneamide (NOR 1), molsidomine (MSD), sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), had no pigment-aggregating action on melanophores, but actively dispersed melanosomes in those cells. Among those reagents, NOR 1, a spontaneous releaser of NO, was the most effective. Inhibitors for nitric oxide synthase (NOS), i.e. Nω-nitro-l-arginine methyl ester (l-NNA), Nω-nitro-l-arginine (l-NAME) and Nω-monomethyl-l-arginine (l-NMMA), showed melanosome-aggregating effects. A membrane-permeable analogue of cyclic guanosine-3′,5′-monophosphate (8-Br-cGMP) was effective in dispersing melanosomes. The sum of these results suggests that NO plays an active role in the elaborate control of color changes in teleosts by dispersing pigment in melanophores via activation of soluble guanylyl cyclase to increase cytosolic levels of cGMP.

Some Mongolian gerbils (Meriones unguiculatus) respond to stimulation by seizures, the pattern of which changes progressively during development. We previously established a seizure-sensitive strain, MGS/Idr, in which all animals exhibit such stimulus-induced seizures. We have now noted that all adults of this strain also show repetitive backward movements of the ears at the beginning of stimulus-induced seizures, although the incidence varies with the individual. We examined whether the cerebral cortex was involved in these movements and found that electrical stimulation of an area of the somatosensory cortex elicited strong backward movement of the ear on the contralateral side, and that unilateral application of bicuculline, a GABA_A receptor antagonist, induced spontaneous repetitive backward movements of the same ear. In this area, sharp waves appeared in the electrocortigram during the repetitive ear movements induced by seizure-inducing stimuli. Unilateral ablation of this area abolished stimulus-induced repetitive movements of the contralateral ear, but had no effects on those of the ipsilateral ear. These results suggest that, in certain types of seizure-susceptible subjects, it may be possible to modify stimulus-induced repetitive movements by manipulating a certain area of the somatosensory cortex which is related to these movements and that this gerbil strain may be useful in research on this subject.

In order to clarify the role of the median (MRN) and dorsal (DRN) raphe nuclei in regulating maternal care (retrieving and licking behavior), radiofrequency lesions or microknife cuts were made in post-partum rats on the day after delivery. Animals were housed individually without pups after the operation. One week after the surgery, maternal behavioral test was carried out daily for 3 days using pups of 2–6 days age. The results demonstrated that rats with MRN lesions or ventral horizontal cuts of the MRN showed extremely low incidence of the maternal behavior, as compared to those in control and sham-operated groups. DRNlesions or dorsal cuts of the MRN had no effect. In locomotor activities measured by the infrared sensor system, there was no difference between the groups. This suggest that the MRN but not DRN plays a critical role in regulating retrieving and licking behaviors and ventral outputs are involved in this function in postpartum rats.

A subtractive cDNA library was made corresponding to mRNAs expressed in the neural complex relative to those expressed in the pharynx of adults of the ascidian Ciona intestinalis. Determination and comparison of expressed sequence tags (ESTs) of a set of 1,527 randomly selected clones demonstrated that they represent 832 independent sequences. Five hundred seventy-two of the clones contained amino-acid-encoding sequences. BLASTX analyses showed that 342 of the 572 clones were strong matches (P<10^–7) to previously identified proteins, while the remaining 230 fell into the “no match” category. Among the clones matching previously identified proteins, about 80 clones represented proteins that are involved in the formation, maintenance of the structure, and function of the nervous system: 22 proteins are associated with signal transduction, five proteins are related to the synapse, 11 to transcription factors, nine to transporters, five to enzymes, and 13 to extracellular matrix and cytoskeletal components, and six to apoptosis. In addition, sequence information for genes associated with the immune system and for genes encoding proteins with interesting functions were obtained. These data provide cues for further studies on genes that are expressed in and function in the ascidian nervous system.

The secretory granules in the androgenic gland of the terrestrial isopod Armadillidium vulgare, which have been indistinct for long time because of vulnerable structures, were revealed by using the rapid-freezing and freeze-substitution method. The fine struture of the androgenic gland is conspicuous by the distribution of numerous particular organelles in the cytoplasm consisting of the endoplasmic reticulum and the Golgi complex, and by having a number of highly organized structures developed between the androgenic gland cells. The structures connect to the intercellular space, which is seen as intercellular canaliculi for exporting the androgenic gland hormone. The plasma membranes near the particular structure of the intercellular canaliculi in the androgenic gland are often specialized to form cellular junctions. The secretory granules including the electron-dense materials, which are supposed to be peptides of androgenic gland hormone, are distributed beside the particular structure of the intercellular canaliculi. Some of the granules are seen to fuse with the plasma membranes. This observation suggests that, in the Armadillidium vulgare, the secretory granules containing androgenic gland hormone are transferred to the extracellular space through the intercellular canaliculi particularly developed for exporting the peptide hormone. This is the first evidence to show the secretory mechanism of the androgenic gland hormone in the Isopoda.

The objective of this study was to determine which intracellular second messenger systems are activated by prothoracicotropic hormone in the prothoracic glands (PGs) of Bombyx mori. Recombinant prothoracicotropic hormone (rPTTH) could stimulate ecdysteroid synthesis and secretion from day 6 PGs of the 5th instar of Bombyx mori within 30 min of in vitro incubation. However, rPTTH did not stimulate any increases in the glandular content of inositol 1,4,5-trisphosphate and cAMP during this short incubation period. Extracellular Ca²⁺ influenced the basal and rPTTH-stimulated ecdysteroid synthesis and release in a dose-dependent manner. The L-type Ca²⁺ channel antagonist, nitrendipine, inhibited the rPTTH-stimulated ecdysteroid synthesis and secretion (IC₅₀ ∼28 μM). The phospholipase C inhibitor, 2-nitro-4-carboxyphenylN, N-diphenylcarbamate, inhibited the rPTTH-stimulated ecdysteroid synthesis (IC₅₀ ∼19 μM). The protein kinase C inhibitor, chelerythrine chloride, inhibited the rPTTH-stimulated ecdysteroid synthesis (IC₅₀∼14 μM). The protein kinase C activator, phorbol-12-myristate 13-acetate (PMA), could stimulate basal ecdysteroid synthesis and secretion (EC₅₀∼1 μM) and its inactive α-isomer (4 α-PMA) was ineffective. The combined results suggest that the PTTH-stimulated ecdysteroid synthesis and release in the PGs of Bombyx is dependent on extracellular Ca²⁺ and the bifurcating second messenger signalling cascade of inositol 1,4,5-trisphosphate and diacylglycerol.

Evidence has accumulated that atrial natriuretic peptide (ANP) plays important roles in sea-water adaptation in eels. However, the roles of the other two natriuretic peptides (CNP and VNP) in osmo-regulation have not been examined yet. In the present study, the effects of homologous ANP, CNP and VNP were compared on plasma Na⁺ concentration (an indicator of plasma osmolality), hematocrit (an approximate indicator of blood volume) and drinking rate in freshwater- and seawater-adapted eels. In seawater eels, ANP and VNP, but not CNP, infused at 5 pmol/kg/min decreased plasma Na⁺ concentration and drinking rate and increased hematocrit. In freshwater eels, ANP and VNP failed to decrease plasma Na⁺ concentration but increased hematocrit to the same extent as in seawater eels. Inhibition of drinking was not detectable in freshwater eels because of little drinking before NP infusions. These results show that the effects of NPs on plasma Na⁺ concentration, drinking rate and hematocrit are mediated by NPR-A, since only ANP and VNP that bind with higher affinity to NPR-A are effective in seawater eels. The mechanisms of regulation of plasma Na⁺ concentration and hematocrit are unknown, but NPR-A is present in the responsible tissues for regulation of hematocrit in both freshwater and seawater eels. However, NPR-A may be absent in the tissues of freshwater eels that are responsible for regulation of plasma Na⁺ concentration.

Three species of marine nemerteans described and illustrated from Queensland and the Great Barrier Reef, Australia, include one new genus and two new species: these are the monostiliferous hoplonemerteans Thallasionemertes leucocephala gen. et sp. nov. and Correanemertes polyophthalma sp. nov. A new colour variety of the heteronemertean Micrura callima is also reported, this species previously only being known from Rottnest Island, Western Australia. A key for the field identification of the marine nemerteans recorded from coastal Queensland and the Great Barrier Reef is provided.

Thais keluo sp. nov. is described from intertidal shores of southwest Taiwan. The new species is differentiated from five other closely related species, namely T. bitubercularis (Lamarck), T. jubilaea Tan and Sigurdsson, T. clavigera (Küster), T. luteostoma (Holten) and T. rufotincta Tan and Sigurdsson, all of which occur in the South China Sea, on the basis of shell, radula and penis morphology. Thais keluo is also distinguished from the latter three species based on allozyme electrophoresis. The shell of T. keluo is characterized by four raised, spiral bands on the last whorl, one or two small, oblique columellar plica(e) on the inner lip, a finely crenate, thin, narrow, reddish-brown outer lip edge and four white, papillate denticles inside the outer lip of the aperture. In males, the penis is curved with a long, simple flagellum. The UPGMA cluster analysis based on 9 enzyme loci revealed that T. luteostoma is more closely related to T. clavigera than to T. keluo n.sp. The Nei's genetic distance (D) obtained between the new species and T. clavigera/T. luteostoma was 0.31, while T. clavigera and T. luteostoma were separated by a distance of 0.16. Thais rufotincta was separated from the other species by a distance of 0.78. In contrast, phylogenetic analysis of morphological data by maximum parsimony suggested that T. luteostoma was more closely related to T. keluo than to T. clavigera. However, both analyses indicated the close relationship amongst T. clavigera, T. luteostoma and the new species in relation to T. rufotincta.

A new species of pagurid hermit crab, Pagurus decimbranchiae, is described and illustrated based on 20 specimens collected from shallow waters of the Pacific coast of Japan ranging from Boso Peninsula to Tanegashima Island. It is quite distinctive in having the rudimentary arthrobranch on the third maxilliped represented by a single bud, however close morphological similarity is found between the new species and P. moluccensis Haig and Ball. Comparisons are also made among other species, including P. boriaustraliensis Morgan, P. sp. cf. boriaustraliensis sensu Rahayu and Komai (2000) and the members of the P. anachoretus group. The present generic assignment of the new species should be considered provisional, as more extensive study is needed to investigate phylogenetic relationships of the new species and the other species of Pagurus.